A Novel T2* GRASE single-shot 3D sequence for BOLD Imaging

The Gradient and Spin Echo (GRASE) pulse sequences has been used primarily for T2 weighted imaging, with the spin echoes of the CPMG sequence utilized for the center of k-space. While to date, T2 contrast has been a fundamental feature of GRASE, it is possible to obtain T2* and BOLD signal contrast mechanisms in GRASE by shifting the central region of k-space from spin echoes to gradient echoes in the echo train. The purpose of this work was to obtain T2* and BOLD contrast in a single-shot 3D GRASE sequence (1). Method: A technique proposed to achieve BOLD contrast in GRASE images is shown in Fig 1. For each 3D k-space partition, ko and the positive side of 2D k-space (k+) are shifted to an earlier time between the 180 refocusing pulses while the negative side (k-) is wrapped into the later time period of the RF pulse interval. Thus the central regions of k-space are positioned onto gradient echoes for T2* weighting which increases BOLD contrast in images. The phase encoding pulses are played out to move the trajectory in the 2D k-space away from the positive edge to the negative edge of k-space or vice versa (refer to k-space diagram in Fig 1). A potential disadvantage of this approach is that it produces a discontinuity of T2* dependent magnitude (M) and phase shifts in k-space, however with the discontinuity at a lower energy non-central region of k-space rather than at k0 , potential artifacts should be greatly reduced. Experiments were performed at 1.5T and 3T in 3 normal individuals, following institutional guidelines. Time course data was obtained with a unilateral left hand grip blocks vs right hand grip blocks, paradigm. At 1.5T the time course data was obtained with 4 blocks of 10 3D images, alternating right and left hand gripping, while at 3T the same unilateral hand grip